Forced plate sensing scheme is widely used in most MEMS applications such as gyroscopes, accelerometers, vibrometers, etc. MEMS micro-g accelerometers for example are in great demand in seismology, military applications etc. MEMS gyroscopes find their applications in GPS and RADAR aided navigation, anti rollover systems, electronic stability control systems and the like.
Inertial grade gyroscopes are in demand at present for space applications since an angular drift of ˜0.001 deg/sec has to be picked up to estimate and correct the drift incase of satellites and GPS guided devices. The present state-of-the-art is mostly based on integrating a MEMS Gyroscope to a highly sensitive capacitance detection circuitry to pick-up changes as low as a few femto-farads in case of rate/tactical Gyros or to pick up capacitance changes in a few atto-farads as in case of inertial grade gyros. The limitation as such is not from the mechanical response of the MEMS Gyro design but from the significant challenges in the electronic sensing circuitry which needs to cater to a reasonable amplification factor while offering very low coupling in terms of parasitic and noise pick-up. Same is the case with milli-g, micro-g sensing accelerometers, vibrometers and the like.
Force sensing is needed in inertial systems to detect acceleration, vibration and angular rate which induce some or the other kind of inertial force (e.g., direct inertial term in accelerometers and coriolis force in gyroscopes). Typically these forces are extremely small and the motion caused by them is what is sensed by these sensors. These motions are in general, rigid body motion of the proof mass leading to a uniformly displacement field that needs to be sensed.
Among the solutions that have been proposed to enhance the sensitivity of force sensors, the following are the most widely used. Sensing based on tunneling concept has been proposed [1], where a Nickel cantilever beam is located above a tunneling tip and tunneling transduction mechanism is used to enhance the sensitivity of the device. A proposal based on magnetic actuation and detection [2] claims a resolution of 0.005°/s, a bandwidth of 70 Hz and a noise floor of <0.5°/s in a 65 Hz bandwidth. The proposal of a novel gyro design which makes use of levitation by electrostatic forces and with no mechanical connection to the substrate is also reported with claims of high sensitivity and dynamically adjustable bandwidth [3]. Built-in amplification by coupling the proof mass of an inertial device as the gate of the FET and exploiting the linear relationship between the gate displacement and the drain current is explored in reference [6].
Limitations in Prior Art:
1. The electronics interface for the model described in [1] requires a complex circuitry involving two servo control loops to maintain a constant oscillation amplitude in the horizontal direction and to maintain a constant tunneling current in the vertical direction. The device has the potential of catering to a rate grade specifications and not beyond.
2. The principle of magnetic actuation and detection employed in [2] is not compatible with standard MEMS and CMOS processing. Also the miniaturization of magnetic actuation is challenging.
3. The control system of the electro statically levitated design [3] comprises of a multi-mode sigma-delta modulator which prove to be a significant overhead in terms of device integration. The feasibility of this principle in forced mass MEMS sensor models and its actual sensitivity when integrated is not yet known.
4. The prevalent way of characterizing small deflection measurements based on differential capacitance sensing (ΔC˜femto or atto farads) requires a complex front-end signal conditioning circuitry [5] and the resolution gets limited by the noise floor of the electronics circuitry [4].
5. Since the FET is biased in saturation regime [6] the drain current is a linear function of in-the plane gate displacement. However this sensing scheme does not offer high amplification factors for displacements of the order of pico-meters.